As is well known in the art of ophthalmology, measuring the intraocular pressure (IOP) of the eye is an important indicator of the health of the eye. Elevated IOP has been associated with progressive damage of the optic nerve known as glaucoma, which, if left untreated, leads to permanent loss of sight.
Various apparatus and techniques have thus been developed to measure IOP. Among the techniques are applanation tonometery, dynamic contour tonometry, transpalpebral diatom tonometry, non-contact tonometry, electronic indentation tonometry, rebound tonometry and digital palpation tonometry.
Applanation tonometry measures approximate intraocular pressure either by the force required to flatten a constant area of the cornea (e.g. Goldmann tonometry) or by the area flattened by a constant force.
In applanation tonometry, a special calibrated disinfected probe attached to a slit lamp biomicroscope is used to flatten the central cornea a fixed amount. Because the probe makes contact with the cornea, a topical anesthetic, such as oxybuprocaine, tetracaine, alcaine, proxymetacaine or proparacaine, is introduced onto the surface of the eye in the form of one or a few eye drops. A yellow fluorescein dye is often also used in conjunction with a cobalt blue filter to aid the examiner in determining the IOP.
Goldmann tonometry is considered to be the gold standard in tonometry, as it is the most widely accepted method of determining “approximate” intraocular pressure. However, as is well known in the art, Goldmann tonometry is an inherently imprecise measurement.
Dynamic contour tonometry (DCT) is a measuring technique that employs the principle of contour matching instead of applanation to eliminate the systematic errors inherent in previous tonometers. These factors include the influence of corneal thickness, rigidity, curvature and elastic properties. DCT is not influenced by mechanical changes, such as those seen in refractive surgery that would otherwise cause error in applanation tonometers.
An exemplar apparatus that employs DCT to measure IOP is the PASCAL Dynamic Contour Tonometer (Ziemer Ophthalmics). The PASCAL uses a miniature pressure sensor embedded within a tonometer tip that is contour-matched to the shape of the cornea. When the sensor is subjected to a change in pressure, the electrical resistance is altered and the PASCAL's computer calculates a change in pressure in accordance with the change in resistance.
The tonometer tip rests on the cornea with a constant appositional force of one gram. This is an important difference from all forms of applanation tonometry wherein the probe force is variable.
In transpalpebral diaton tonometry, a diaton tonometer is employed to measure intraocular pressure through the eyelid. It is typically regarded as a simple and safe method of ophthalmotonometry. Transpalpebral tonometry requires no contact with the cornea, therefore sterilization of the device and topical anesthetic drops are not required.
Non-contact tonometry or air-puff tonometry uses a rapid air pulse to applanate the cornea. Corneal applanation is detected via an electro-optical system. Intraocular pressure is estimated by detecting the force of the air jet at the instance of applanation.
Modern-day non-contact tonometers have been shown to correlate very well with Goldmann tonomtery measurements and have thus generally been considered a fast and simple way to screen for high IOP. Further, since non-contact tonometry is accomplished without the instrument contacting the cornea the potential for disease transmission is reduced.
Electronic indentation tonometry employs a Tono-Pen, i.e. a portable electronic, digital pen-like instrument that determines IOP by making contact with the cornea. Electronic indentation tonometry is especially useful for very young children, patients unable to reach a slit lamp due to disability, patients who are uncooperative during applanation tonometry, or patients with cornea disease in whom contact tonometry cannot be accurately performed.
In palpation tonometry, also known as digital palpation tonometry, measuring intraocular pressure is performed by gently pressing the fingertips of both index fingers onto the upper part of the bulbus through the eyelid. This technique requires medical experience and results in an estimation of the level of intraocular pressure based on the skills of the ophthalmologist.
A major drawback associated with each of the noted techniques is that each technique requires professional assistance to measure IOP.
A further drawback associated with each of the noted techniques is the need for topical anesthesia and complex instrumentation to measure IOP, which makes multiple daily measurements impractical.
It would thus be desirable to provide improved systems and methods for measuring IOP that overcome the disadvantages and drawbacks associated with conventional systems and methods for measuring IOP.
It is therefore an object of the present invention to provide systems and methods for measuring IOP that can be performed by the patient without professional assistance.
It is another object of the present invention to provide systems and methods for measuring IOP that do not require anesthesia.
It is another object of the present invention to provide systems and methods for determining outflow of aqueous humor from an eye as a function of the measured forces exerted by the eye.